Now to the suspects, there are new details tonight about who they are, also new questions about what exactly the older brother did on a six-month trip to russia and why the fbi investigated him two years ago and then cleared him. Abc's senior justice correspondent pierre thomas has been on that angle all day. Pierre, good evening to you. Good evening, dan. With the manhunt now over, law enforcement is looking for a motive behind the bombings? Was anyone else involved? training? Immigrants from the russian caucuses, the brothers came with their parents to the u.S. As children. Seeking refugee status. They thrived attending top boston schools. Older brother, tamerlan, 26, star athlete, golden glove boxer, pianist. Younger brother dzhokhar had just become a u.S. Citizen, ironically, on septeer 11th of last year. Once a lifeguard, he was an all-state wrestler and college student. I certainly can't put it together that he was a person firing a weapon and potentially killing people. ReporteT NIGHT THE FBI Acknowledged it interviewed tamerlan in 2011 based on a tip from the russians. The russians said he was a radical islamist, preparing to leave the u.S., Possibly for a trip to the caucuses. After looking into tamerla phone records, websites he visited and associates, the fbi found no ties to terrorists. By law, fbi officials said, monitoring had to stop, barring new negative information. Too many people in this country, just like him, that are touching extremist websites, that are espousing things that aren't particularly kind to americans, but that's not against the law here. Reporter: Tamerlan's father said his sons couldn't have planned the attack, because they knew the fbi was watching them. But they apparently were not. And never knew that a dark side of the two brothers had begun to emerge. Tamerlan was charged in cambridge with beating his former girlfriend. He started posting violent videos, urging ji had. And dzhokhar began writing bizarre tweets. A decade in america already, i want out. Authorities neveis. Tamerlan traveled to russia from -- for six months last year. The fbi wants to know exactly who he saw and what he was doing, dan. Let me get back to the investigation into the older brother. Are they not expecting any sense of frustration or remorse over having missed what could have been a rea opportunity here? They'll do a postmortem, but

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